1. Field of the Invention
The present invention relates to a process for the formation of a pattern by which a pattern can be formed under optimum light source conditions giving little disadvantage of the secondary peak even if for example a phase shift mask is used and to an exposure apparatus.
2. Description of the Related Art
At present, in the research and development of semiconductor integrated circuits, devices of a design rule of a sub-half micron area are being researched and developed. In the development of these devices, photolithographic technology is indispensable. It is not too much to say that the resolution performance of the exposure apparatus used in this photolithographic technology, i.e., the so-called "projection aligners" (reduction, projection, and exposure apparatuses), governs the success of the research and development of semiconductor devices and the possibility of their mass production.
The resolution performance of the projection aligners has been improved by making the NA of the reducing and projecting lens larger or making the exposure waveform shorter based on the Reyleigh equation. In the fabrication of a semiconductor device, however, there are step differences caused due to the topography, wafer flatness, etc. of the semiconductor device, therefore not only the resolution performance, but also the ensurement of the depth of focus are important parameters. The dimensional precision of the resist pattern in the photolithographic process in the fabrication of a semiconductor device is generally .+-.15 percent. In an actual device, as shown in FIG. 1, unevenness always occurs on the surface of the semiconductor substrate S. For example, a projecting portion In of as polycrystalline silicon etc. exists. As a result, the pattern of the resist PR is not formed at the same focal plane. For this reason, the dimensions of the pattern of the resist PR become different between the upper portion and lower portion of the step difference. Of course, this becomes conspicuous as the pattern becomes finer where steppers having the same waveform and same numerical aperture are used. This tendency is commonly seen in all types of resists.
The depth of focus becomes smaller primarily in proportion to the exposure waveform and inversely in proportion to the square of the NA. In a mass production process, a depth of focus of about 1.5 .mu.m is necessary. For this reason, there is a limit to which both of the resolution performance and depth of focus which are required can be satisfied.
FIGS. 2A and 2B are views of the NA dependency when the resolution performance of the depth of focus (DOF) in KrF excimer laser lithography, which is the latest exposing process, is used as a parameter. As understood also from the figures, the highest resolution to be obtained after satisfying the required depth of focus of 1.5 .mu.m is about 0.35 .mu.m. Accordingly, it is extremely difficult to resolve a line width of 0.35 .mu.m or less while having a depth of focus of 1.5 .mu.m or more. Some technologies for increasing the depth of focus are necessary.
In order to respond to such requirements, a halftone type phase shift method has been proposed in recent years. This exposure process is an extremely useful process for improving the degree of resolution and depth of focus of an isolated pattern such as contact holes. In the halftone type phase shift process, as shown in FIG. 3, semi-transparent Cr.sub.x O.sub.y, Si.sub.x N.sub.y, SiO.sub.x N.sub.y, Mo.sub.x Si.sub.y films, etc. which have a transmittance of about several percentages to about 20 percent with respect to the light for exposure, that is, which transmit a minute amount of light for exposure therethrough, are used as the halftone film 2 corresponding to a dark portion 1. In a bright portion 3, both of the film 2 and the transparent substrate (on which the recessed portion 5 is formed) or only the film 2 is etched and made to act as a mask. At this time, by setting the phase difference between the bright portion 3 and the dark portion 1 formed by the semi-transparent film to 180.degree., as shown in FIG. 4B, the gradient of the distribution of the light intensity in the isolated pattern (for example a hole pattern of 0.6 .gamma./NA) can be made sharp. Note that, FIG. 4A is a view of the distribution of the light intensity in an isolated pattern using a chromium mask in related arts.
FIG. 5 is a view of a result of experiments to show that the DOF is greatly improved when using a phase shift mask so as to form an isolated contact hole. The black dots in FIG. 5 is a view of the case where the phase shift mask is used, and the white dots show the case where a chromium mask of related arts is used.
In the design of this phase shift mask, the transmittance of the halftone film 2 shown in FIG. 3 is an important element. Namely, in order to make the gradient of the distribution of the light intensity in the isolated pattern sharper, it is sufficient to raise transmittance of the halftone film 2. However, by raising the transmittance, the light blocking effect by the halftone film 2 is weakened, and the whole surface of the resist is exposed.
Also, usually, at the formation of the pattern, on the two sides of the position of the desired pattern in the distribution of the light intensity, irrespective of the light blocking position, as shown in FIG. 6, a secondary peak called a side lobe is caused due to the proximity effect. The secondary peak is enhanced by raising the halftone transmittance. As shown in FIG. 7, even in so-called completely isolated contact holes in which the adjoining patterns are spaced apart from each other by 3W or more where the design dimension of for example the hole pattern 6 is W, the peripheral portions end up with a "gouged" shape (reference numeral 8 part). In the shape shown in FIG. 7, there is a concern that the diameter of the contacts will be enlarged in the etching step.
Further, when it is intended to apply the halftone phase shift mask process to a so-called repeated pattern portion having a high pattern density, the secondary peak is emphasized by the mutual interference of adjoining patterns, that is, the mutual proximity effect in a so-called repeated pattern portion having a high pattern density, and therefore becomes more conspicuous.
Accordingly, when it is intended to form the device pattern by using the halftone phase shift mask process, a design and CAD step must be carried out while sufficiently considering the distance between patterns. An enormous load is placed on the design and CAD step, and practical use is obstructed.
In order to suppress the resolution of the secondary peak, it is sufficient if the light intensity of the secondary peak is lowered to an extent that the resolution does not occur. The light intensity of the secondary peak depends upon the light intensity of the center portion of the light source. Therefore, a process of suppressing the resolution of the secondary peak by lowering the light intensity of the center portion of the light source is proposed.
In this way, it was confirmed by experiment that, according to the process of exposure lowering the light intensity of the center portion of the light source and using a phase shift mask, the secondary peak is not resolved by a light source having a distribution of the light intensity of a usual Gaussian distribution even if the pattern interval of the contact holes is narrower than that of the exposure process.
For example, FIG. 8B is a view of an example in which a pattern of contact holes having an inner diameter of 0.30 .mu.m is formed on the substrate by a KrF excimer laser stepper having an NA of 0.45 by using a effective light source having a low light intensity at the center portion and halftone phase shift mask. In this experiment, chemical amplification type positive resist (WKR-PT2) was used as the resist. Contact hole patterns were formed for patterns in which the ratio of the inner diameter of the contact holes and the interval between them (duty ratio) was changed to 1:3, 1:1.5, and 1:1. The results of SEM photographs are shown in FIG. 8B.
Also, for comparison, contact hole patterns were formed under three types of conditions in the same manner except that the distribution of intensity of light irradiated onto the fly-eye lens was made the Gaussian distribution shown in FIG. 8A. Further, for comparison, contact hole patterns were formed under three types of conditions in the same manner except that the distribution of intensity of light irradiated onto the effective light source was made the distribution of the ring belt illumination (the light intensity of the center portion was set to 0). The results by observation by SEM photographs are shown in FIG. 8C. It was confirmed from the results of these experiments that the secondary peak was almost not isolated by the exposure having a light source with a distribution where the light intensity was weak at the center portion (FIGS. 8B and 8C) even if the interval between the contact holes became narrow.
Where it is intended to form a pattern of isolated contact holes, however, as shown in FIG. 9, in the combination (.smallcircle. in the figure) of the ring belt illumination and the halftone phase shift mask, the DOF is lowered (the width of the focus becomes narrow in the figure) compared with the combination between the usual illumination and the half tone phase shift mask (.quadrature. in the figure).
Namely, the resolution of the secondary peak can be suppressed by lowering the light intensity of the center portion of the light source, but when it is lowered too much, the effect of the halftone phase shift mask is reduced. Also, as shown in FIGS. 8A to 8C, it is seen that the degree of resolution of the secondary peak differs depending upon the interval of the patterns. Accordingly, it is necessary to optimize the conditions such as the distribution of the light intensity of the light source so as to suppress the resolution of the secondary peak, but not reduce the effect of the phase shift mask.